Thermoplastic elastomers are used extensively as the polymeric component of hot melt and pressure sensitive adhesives. These elastomers are multicomponent polymers that contain a hard, semi-crystalline, or nonflexible, phase dispersed in a soft, amorphous, elastomeric phase. The polymers are molten above the glass transition temperature or melt temperature of the hard polymeric phase, and aggregate below these temperatures into domains that behave like physical crosslinks. The elastomeric phase constitutes the larger portion of the polymer and becomes the continuous phase, which gives the composite polymer the flexibility and elongation of a rubber.
Commercially available thermoplastic elastomers generally fall into three types: styrene-elastomer-styrene triblock copolymers, such as, styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers and their corresponding hydrogenated versions; thermoplastic polyurethanes; and thermoplastic segmented polyesters, such as poly(tetramethylene glycol) as the elastomeric component and poly(tetramethylene terephthalate) as the hard component.
These commercially available thermoplastic elastomers have certain drawbacks in chemical and mechanical properties, and they can be difficult to manufacture. The triblock copolymers are made by anionic polymerization, which requires the exclusion of oxygen and moisture; the hard domains are glassy rather than crystalline; the viscosities are high because the polymers are linear and not branched; and the melt (or softening) points are low, which can give inadequate heat resistance or green strength. The polyesters have many of the same problems as the triblock copolymers and are prepared at high temperatures and high vacuum. The polyurethanes are not thermally stable. In addition, many of these polymers are derived from petroleum products and are cost dependent on the pricing of those products; moreover, they are very resistant to degradation.
Processes for making polylactide graft copolymers have been described in the literature. An article by Eguiburu, Berridi and Roman, entitled "Functionalization of poly(L-lactide) macromonomers by ring-opening polymerization of L-lactide initiated with hydroxyethyl methacrylate-aluminum alkoxides," published in POLYMER, volume 36, number 1, pages 173-179 (1995), reported the preparation of acrylic macromonomers of poly(L-lactide) by the ring-opening polymerization of L-lactide, initiated by acrylic functionalized aluminum alkoxides. Specifically, the initiators were mono- and tri-hydroxyethyl methacrylate-aluminum alkoxides. The polymerization was carried out in toluene at 60.degree. C. and yielded methacrylate terminated poly(L-lactide) macromonomers that were reported to be copolymerizable with other vinyl or acrylic monomers to provide graft copolymers.
An article by Barakat, DuBois, Jerome, Teyssie and Goethals, entitled "Macromolecular Engineering of Polylactones and Polylactides. XV. Poly(D,L)-lactide Macromonomers as Precursors of Biocompatible Graft Copolymers and Bioerodible Gels", published in The Journal of Polymer Science: Part A:
Polymer Chemistry, Volume 32, pages 2099-2110 (1994), reported the ring-opening polymerization of L-lactide, initiated by acrylic functionalized aluminum alkoxides, in toluene at 70.degree. C. The poly(L-lactide) macromonomer could be synthesized to contain acrylate functionality at either one or both ends of the macromonomer. The acrylate-functionalized macromonomers were then copolymerized with other acrylate comonomers.
These processes suffer from several disadvantages: the acrylate macromonomers are less reactive than the normal acrylate monomers and require more stringent reaction conditions for copolymerization; the higher reaction temperatures needed to prepare the macromonomers can result in uncontrolled polymerization; the aluminum alkoxide initiators give aluminum salt by-products that must be washed from the polylactide/acrylate product; and the functionalized polylactide polymers are soluble only in relatively noxious solvents.
The present invention provides polylactide graft copolymers in which the grafting of the polylactide occurs after the formation of the polymer backbone.